In the electric power steering apparatus for energizing assist force by rotation force of the motor in a steering apparatus of the automobile or the vehicle, the assist force is energized to a steering shaft or a rack shaft by a transmission mechanism of a gear or a belt, etc. through a speed reduction gear with respect to driving force of the motor. An inverter, etc. are used in a motor driving circuit to supply a current to the motor so as to generate a predetermined desirable torque by this motor.
Here, FIG. 1 shows a basic construction of an electric power steering apparatus disclosed in a Japanese patent literature (JP-A-8-142884). FIG. 2 shows the details of a motor driving circuit within this electric power steering apparatus. In FIG. 1, torque detected by a torque sensor 103 is inputted to a phase compensator 121, and a torque command value is calculated. Next, the torque command value is inputted to a current command calculating element 122. A vehicle speed detected by a vehicle speed sensor 112 is added and a current command value Iref is calculated by the current command calculating element 122. In this control, feedback control is adopted. A current Imes of a motor 110 as a control object is detected by a motor current detecting circuit 142, and is fed back to a comparator 123. The current Imes is then compared with the current command value Iref, and an error is calculated. So-called proportion integration control of this error is performed by a proportion calculating element 125 and an integration calculating element 126. The current command value is inputted to a differentiating compensator 124 for improving a transient response. The respective outputs of the differentiating compensator 124, the proportion calculating element 125 and the integration calculating element 126 are added by an adder 127, and a current control value E is calculated. A motor driving circuit 141 supplies a current to the motor 110 on the basis of the current control value E as an input value. A battery 114 is an electric power source of the motor driving circuit.
FIG. 2 shows details of the motor driving circuit 141. The motor driving circuit 141 is comprised of an inverter unit constructed by FET as a switching element, and a gate control unit for controlling the operation of gate of FET. The inverter unit comprises an H-bridge which is constructed by a up-and-down arm constructed by FET1 and FET3, or a up-and-down arm constructed by FET2 and FET4. In the gate control unit, the current control value E is inputted to a converting unit 130, and a timing signal with respect to each FET is generated and inputted to gate driving circuits 133a, 134a, 133b, 134b. Thus, a gate signal able to operate gate of FET is generated. However, the timing signal generated by the converting unit 130 is not directly inputted to the gate driving circuits 134a and 134b, but is respectively inputted to a dead time circuit 131 and a dead time circuit 132 because of the following reasons.
Each up-and-down arm constituting the inverter unit, e.g., FET1 and FET3 are alternately repeatedly turned on and off. Similarly, FET2 and FET4 are alternately repeatedly turned on and off. However, the FET is not an ideal switch. Therefore, no FET is turned on and off in a moment in accordance with indication of the gate signal, but a turn-on time and a turn-off time are required. Therefore, when the indication of turning-on FET1 and the indication of turning-off FET3 are simultaneously performed, a problem exists in that FET1 and FET3 are simultaneously turned on and the up-and-down arm is short-circuited. Therefore, when an off-signal is given to the gate driving circuit 133a so as not to simultaneously turn-on FET1 and FET3, an on-signal is not immediately given to the gate driving circuit 134a, but is given to the gate driving circuit 134a by putting the pause of a predetermined time called a so-called dead time by the dead time circuit 131. Thus, the up-and-down short circuit of FET1 and FET3 is prevented. This also similarly hold true in FET2 and FET4.
However, the existence of this dead time becomes a cause generating the problem of torque insufficiency and a torque ripple in the control of the electric power steering apparatus. This problem will next be explained in detail.
First, FIGS. 3A to 3D show the relation of the dead time, the turn-on time and the turn-off time. In FIGS. 3A to 3D, a signal K is basically set to on and off signals with respect to FET1 and FET3. However, in reality, a gate signal K1 is given to FET1, and a gate signal K2 is given to FET2. Namely, the dead time Td is secured. A phase voltage constructed by FET1 and FET2 is set to Van. Even when the on-signal due to the gate signal K1 is given, the FET is not immediately turned on, but is turned on after a turn-on time Ton is required. On the other hand, even when the off-signal is given, the FET is not immediately turned off, but is turned off after a turn-off time Toff is required. Vdc is an electric power voltage of the inverter.
Accordingly, a total delay time Ttot is expressed by the following expression 1.Ttot=Td+Ton−Toff   [Expression 1]
Here, the turn-on time Ton and the turn-off time Toff are changed by kinds, capacities, etc. of used FET and IGBT, etc. Further, the dead time Td is generally a value greater than the turn-on time Ton and the turn-off time Toff.
Next, an influence affected by this dead time Td will be explained.
First, there is the following influence in the influence with respect to the voltage. As shown in FIGS. 3A to 3D, the actual gate signals K1 and K2 with respect to an ideal gate signal K differ from the gate signal K by the influence of the dead time Td. Therefore, distortion is generated in the voltage. However, a value ΔV of this distortion voltage is shown in expression 2 when the direction of a motor current is positive (when the current is directionally flowed from the electric power source to the motor). The value ΔV is shown in expression 3 when the direction of the current is negative (when the current is directionally flowed from the motor to the electric power source).−ΔV=−(Ttot/Ts)·(Vdc/2)  [Expression 2]
Where, Ts is an inverse number Ts=1/fs of a PWM frequency fs when the inverter is PWM-controlled.ΔV=(Ttot/Ts)·(Vdc/2)  [Expression 3]
When the above expressions 2 and 3 are represented by one expression, the following expression 4 is formed.ΔV=−sign(Is)·(Ttot/Ts)·(Vdc/2)  [Expression 4]
Here, sign(Is) represents the polarity of the motor current.
It is derived from the expression 4 that the influence of the dead time Td with respect to the distortion voltage ΔV greatly appears as the frequency fs is high and the electric power voltage Vdc is small.
The influence of the dead time Td with respect to the voltage distortion has been explained. However, with respect to the current or torque, there is an unpreferable influence affected by the dead time Td. With respect to the current distortion, when the current is changed from the positive current to the negative current or is changed from the negative current to the positive current, a phenomenon (zero clamping phenomenon) for fixing the current to the vicinity of zero is generated by the dead time Td. This is because there is a tendency intended to maintain the current to be zero by a reduction in voltage due to the dead time Td since load (motor) is inductance.
Further, output deficiency of torque and an increase of the torque ripple appear as the influence of the dead time Td with respect to torque. Namely, the current distortion causes a higher harmonic wave of a low order and this generation results in the increase of the torque ripple. Further, the output deficiency of torque is generated since the real current influenced by the dead time Td becomes smaller than the ideal current.
Various countermeasures, so-called dead band compensation has been considered to prevent such an unpreferable influence of the dead time Td. Its basic idea is to compensate the distortion voltage ΔV represented by expression 4. Accordingly, a correction is performed by a correction voltage Δu represented by the following expression 5 to compensate expression 4.Δu=sign(Is)·(Ttot/Ts)·(Vdc/2)  [Expression 5]
Here, it is a problem that polarity sign(Is) of the current Is can not be correctly detected. When the polarity of the current Is is measured, it is difficult to correctly measure the polarity of the current Is by noises of the PWM control and the above zero clamping phenomenon of the current.
In many conventional dead band compensations (e.g. disclosed in literature 1 (Ben-Brahim, The analysis and compensation of dead-time effects in the three phase PWM inverters, Proceedings of the IEEE-IECON98, Volume 2, pages 792-797)), methods are complicated and the addition of hardware is required. Further, no countermeasure considering a change of a load current such as the motor current, etc. is taken.
Therefore, the dead band compensation for preventing the up-and-down arm short circuit of the inverter causes distortions of the motor voltage and current or the output deficiency of torque and the increase of the torque ripple. An improvement countermeasure of this dead band compensation is conventionally complicated, and the addition of hardware is caused. Further, this compensation is imperfect dead band compensation in which no influence of the motor load current is considered.
The present invention is made from the above situations, and its object is to provide a controller of an electric power steering apparatus in which distortions of motor voltage and current and torque ripple are small by using dead band compensation having a simple construction and also considering influence of motor load current.